Surface mountable light emitting device

ABSTRACT

This invention relates to a surface mountable light emitting device in which the lead frame is exposed over a substantial portion of the underside of the device so as to allow greater thermal conductivity to any device on which it may be mounted. The LED provides the lens and a molded body to encapsulate the lead frame and an electrical contact in a single molding step while the lead frames and further contacts are arranged in a suitable array. The lens couples the luminous output of a light-emitting diode (LED) to a predominantly spherical pattern comprises a transfer section that receives the LED&#39;s light within it and an ejector atop it that receives light from the transfer section and spreads it spherically. Applications may include, but are not limited to, household light bulbs and car headlights.

BACKGROUND OF THE INVENTION

This invention relates to a surface mountable light emitting device,particularly, a surface mountable light emitting device having a highpower capacity.

Conventional light emitting diodes provide a light emittingsemiconductor chip within a metal cup, a lead wire to a further contacton the chip, a bullet lens over the structure and a body around thestructure. Often the body would be formed integrally with the bulletlens. Both the cup and the lead wire are attached to legs extending froman underside of the body for connection through a printed circuit boardinto a suitable circuit.

The manufacture of products that utilize large numbers of light emittingdiodes may favor the use of surface mountable devices. The attachment ofmany such LEDs to, for example, a printed circuit board holding thedriving circuitry can be achieved considerably more economically byautomated machines. If such a machine can operate on a single side ofthe printed circuit board to place and secure the LED, significantsavings may be made, and the reverse side of the printed circuit boardcan be left free for the provision of the driving circuitry. All of thisrequires a surface mountable device that avoids the traditionalplacement of the legs of the LED through a printed circuit board andsoldering on the reverse side of the board.

A variety of methods have been attempted to achieve a suitable surfacemountable light emitting device. Usually, such methods have involved theprotrusion of the lead wire and a connection to the lead frame at theside of the device for attachment to the surface on which it is to bemounted. Although surface mountable, such connections are arrangedaround a perimeter of the device, which limits the density at which theymay be mounted on the surface.

A further problem with light emitting devices occurs more permanentlywith high power devices. An LED running at high power, such as at onewatt generates a significant amount of heat. This heat can deterioratethe performance of the LED or, over time, lead to the destruction orburn out of the LED.

Although the heat may be dissipated by the surrounding apparatus, thisstill requires the transfer of the heat from the source, to outside ofthe LED. The legs extending from the body of the LED provide arelatively small thermal pathway, and do not allow sufficient heatdissipation to allow high power units on the order of one watt.

A yet further difficulty in the subject art arises in the manufacture ofLEDs. It is difficult to provide a process that allows easy manufactureof LEDs with a minimum of components while assuring the requirements,e.g., greater heat dissipation, of high power units are met.

Also, conventional LEDs are optically unsuitable for direct installationinto devices such as headlamps or flashlights that use parabolicreflectors. This is because the bullet lenses used form a narrow beamthat completely misses a nearby parabolic reflecting surface. Using,instead, a hemispherically emitting non-directional dome, centered on aluminous LED die, gives a maximum spread commercially available, aLambertian pattern. Since θ for a typical parabolic flashlight reflectorextends from 45° to 135°, an LED with a hemispheric pattern is stillmismatched with respect to a parabolic reflector because the LED'semission falls to zero at only θ=90°. This results in a beam that isbrightest on the outside edges and completely dark halfway in to itscenter. Worse yet, even this inferior beam pattern from a hemisphericLED requires that the LED be held up at the parabola's focal point,several millimeters above the socket wherein a conventional incandescentbulb would be installed.

There is thus a need in the art for an effective and optically suitablesurface mountable light emitting device (LED) that avoids thetraditional placement of the legs of the LED through a printed circuitboard and soldering on the reverse side of the printed circuit board,provides sufficient heat dissipation, allows easy manufacture withminimum components, ensures the requirements of high power usage aremet, and is optically suitable for direct installation into devices thatuse parabolic reflectors as replacements for tungsten filament lightbulbs.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as wellas other needs by providing a surface mountable light emitting devicethat avoids the traditional placement of legs of the LED through aprinted circuit board, and soldering of the legs to the printed circuitboard on the reverse side of the printed circuit board, providessufficient heat dissipation, allows easy manufacture with minimumcomponents, ensures the requirements of high power usage are met, and isoptically suitable for direct installation into devices that useparabolic reflectors and replacement of tungsten filament light bulbs.

In one embodiment, the invention can be characterized as a high power,surface mountable light emitting device comprising a light emittingsemiconductor chip, a thermally and electrically conductive lead frameconnected to said chip and exposed over a substantial portion of theunderside of the device, a lead wire from said chip to a contact exposedat least partially on a side of said device and a lens over said chip.

The lens comprises a lower transfer section and an upper ejector sectionsituated upon the lower transfer section. The lower transfer section isoperable for placement upon the light emitting semiconductor chip andoperable to transfer the radiant emission to said upper ejector section.The upper ejector section is shaped such that the emission isredistributed externally into a substantial solid angle.

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription of the invention and accompanying drawings, which set forthan illustrative embodiment in which the principles of the invention areutilized.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a side cross-sectional view of an optical device according toan embodiment of the present invention;

FIGS. 2A though 2F are side cross-sectional, top perspective, bottomperspective, side, top planar, and side elevational views, respectively,of the lens of the optical device of FIG. 1 according to an embodimentof the present invention;

FIGS. 3A and 3B are side perspective and bottom planar views,respectively, of the optical device of FIG. 1;

FIG. 3C is a side perspective view of an optical device according to analternative embodiment of the present invention;

FIG. 4A is a side perspective view of an optical device according to analternative embodiment of the present invention;

FIG. 4B is a bottom planar view of the device of FIG. 4A;

FIG. 5A is a schematic of a driving circuit of an optical deviceaccording to an embodiment of the present invention;

FIG. 5B is a schematic of a driving circuit of an optical deviceaccording to an alternative embodiment of the present invention;

FIG. 5C is a schematic of a driving circuit of an optical deviceaccording to an alternative embodiment of the present inventionutilizing an integrated circuit;

FIG. 6 is a side cross-sectional view of a light bulb integrating thedevice of FIG. 1 according to an embodiment of the present invention;and

FIG. 7 is a partial side cross-sectional view of an optical deviceaccording to an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIES

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The scope of the invention should be determined withreference to the claims.

Referring to FIG. 1, shown is a side cross-sectional view of an opticaldevice according to an embodiment of the present invention.

Shown is a light emitting device 1 having a main body portion 2according to that described in Hong Kong patent application No.03104219.4 filed Jun. 12, 2003 for A SURFACE MOUNTABLE LIGHT EMITTINGDEVICE AND METHOD OF MANUFACTURE, the entirety of which is herebyincorporated by reference, and lens 10 according to that described inU.S. patent application No. 60/470,691 of Minano et al., for OPTICALDEVICE FOR LED-BASED LIGHT-BULB SUBSTITUTE, filed May 13, 2003 (docketNo. 3084.003), and U.S. Pat. No. ______ of Minano et al., for OPTICALDEVICE FOR LED-BASED LIGHT-BULB SUBSTITUTE, filed Jun. 12, 2003 (docket3084.008), the entirety of which is also hereby incorporated byreference. The main body portion 2 has a lead frame 3, a further contact4, an LED semiconductor chip 6, electrical connection 7, and portions ofa transparent optical molded compound 8, 9. A lens 10 comprises a lowertransfer section 11, an upper ejector section 12 and a conicalindentation 13.

The lead frame 3 is located in the main body portion 2. The LEDsemiconductor chip 6 is mounted on the lead frame 3. A further contact 4to provide the path for current through the chip 6 is also provided andattached to the semiconductor chip 6 by an electrical connection 7 suchas a lead bonding wire. Preferably the LED semiconductor chip is asingle bond pad LED, but may also be a double bond pad LED (which wouldrequire an additional cathode bonding wire). The lens 10 is providedover an upper surface to encapsulate the semiconductor chip 6 andprovide preferred optical characteristics.

As also shown in this particular embodiment, the lens 5 and the mainbody portion 2 may be provided in a single molding step as integrallymolded portions from the same material using a transparent opticalmolded compound. Alternatively, the lens 10 may attach to the device 1using optical grade glue. To act as a lens, material used to form thelens 10 should be substantially transparent, although not necessarilycompletely transparent as there may be some desire to adapt the opticalcharacteristics of the output of the semiconductor chip with the lens10. The molding of the lens 10 and the main body portion 2 in a singleintegral structure allows the transparent optical molded compound to bekeyed into the lead frame 3 and further contact 4 by the portions of thetransparent optical molded compound 8 and 9. This helps secure the leadframe 3 and further contact 4 in place in the final device with minimalneed for adherence between the transparent optical molded compound andthe metal of the lead frame 3 and the further contact 4.

The semiconductor chip 6 is recessed into a recess within the lead frame3 such that the sides of the recess act as a reflector. The purpose ofsuch a reflector around the semiconductor chip 6 is to redirect lightthat may be emitted from the sides of the semiconductor and reflect thelight generally out through the transparent optical molded compound 5upwardly from the semiconductor chip 6.

Example applications include, but are not limited to, replacement ofincandescent lights or other luminaire light sources that arenon-directional and pointed like an LED, replacement of a flashlightbulbs, use as exterior and interior automotive lights, use in miniatureindustrial light bulbs, and any other lighting applications that requireuse of a pseudo filament to mimic traditional luminaries. This mayinclude, for example, marine control panels or dashboard lights, avioniccockpit panel lights and other interior lighting that utilizes miniaturelight bulbs.

Referring next to FIGS. 2A through 2F, shown are side cross-sectional,top perspective, bottom perspective, side, top planar, and sideelevational views, respectively, of the lens 10 of the optical device 1of FIG. 1 according to an embodiment of the present invention.

Shown are the lens 10, lower transfer section 11, upper ejector section12 and conical indentation 13.

The lens 10 comprises a lower transfer section 11 and an upper ejectorsection 12. The lens 10 is a substantially transparent solid in thegeneral shape of a prolate ellipsoid and is a single piece of atransparent optical molded material such as acrylic or polycarbonate.The lens 10 is preferably a rotationally symmetric shape, larger inheight than in diameter, but need not be so (e.g., a free form shape).The upper ejector section 12 is cylindrical, with a conical indentation13 on top, having a core angle of approximately 80°.

The lower transfer section 11 uses internal reflection to relocate thedevice's 1 emission upward to a parabola's focal point. The upperejector section sends the transferred light out (to a parabolicreflector, for example), sideways and downward at angles to the axisextending all the way to at least 135°, or a little more, (measuredrelative to a central axis of the ejector section, back toward thesemiconductor chip 6) depending upon the reflector. At least half theejected light should be at angles over 45° (measured relative to acentral axis of the ejector section back toward the semiconductor chip6), in order to illuminate a reflector (not shown) and form asufficiently intense collimated beam.

In order to avoid an external reflective coating on the surface of thetransfer section 11, its geometry must promote total internalreflection. This is why polycarbonate, with its higher refractive index(1.5855), is preferable to acrylic (1.492). Its correspondingly smallercritical angle, θc=sin−1(1/n), of 39.°103 vs. 42.°1, reduces the heightof the transfer section from 23.5 mm to 11.6 mm.

Referring next to FIGS. 3A and 3B, shown are side perspective and bottomplanar views, respectively, of the optical device of FIG. 1.

Shown are the light emitting device 1, the main body portion 2, the leadframe 3, the further contacts 4, the lens 10, the lower transfer section11, the upper ejector section 12 and the conical indentation 13.

The further contact 4 can be seen exposed on the side of the main bodyportion 2. It can also be seen in FIG. 3B that the lead frame 3 isexposed over a substantial portion of the underside of the device 1.This exposure of a large surface area of the lead frame 3 on theunderside of the light emitting device 1 allows substantial heat to bedrawn directly from the lead frame 3 into a surface on which the leadframe 3 may be mounted.

Referring next to FIG. 3C shown is a side perspective view of an opticaldevice 1 according to an alternative embodiment of the presentinvention. Shown is the light emitting device 1, main body portion 2,further contact 4 and lens 16 comprised of an off-axis ellipsoidaltransfer section 17 and a spherical, diffusive ejector section 18according to that described in U.S. patent application No. 60/470,691 ofMinano et al., for OPTICAL DEVICE FOR LED-BASED LIGHT-BULB SUBSTITUTE,filed May 13, 2003 (docket No. 3084.003), and U.S. Pat. No. ______ ofMinano et al., for OPTICAL DEVICE FOR LED-BASED LIGHT-BULB SUBSTITUTE,filed Jun. 12, 2003 (docket 3084.008).

The outer surface of the ejector section 18 has diffusivecharacteristics, (i.e. surface features that cause light to diffuse), sothat each point on the ejector section 18 has a brightness proportionalto the light received from the transfer section 17. The advantage ofthis kind of ejector section 18 is that the multiple wavelengths, forexample, from a tricolor LED are mixed before they leave the ejectorsection 18. In the non-diffusive ejector section 12 discussed above,which is non-diffusive, the color mixing may be incomplete, leading tocoloration of the output beam of a parabolic reflector. The lens 16comprises an off-axis ellipsoidal lower section 17 and an upperspherical ejector section 18. The upper spherical ejector section 18 issmaller than the transfer section 17 (i.e., having a smaller diameterthan a middle diameter of the transfer section 17). Due to the smallerupper spherical ejector section's size it radiates less in angles beyond90° than if the upper spherical ejector section 18 were larger than thetransfer section 17. Such a upper spherical ejector section will alsoact to mix the colors of the red, green, and blue source chips within anLED light source.

Referring next to FIGS. 4A and 4B, shown are a side perspective andbottom planar views, respectively of an optical device according to analternative embodiment of the present invention.

Shown is the light emitting device 19, the main body portion 2, the leadframe 3, the further contacts 4, the LED components 21, 22, 23 and thelenses 10.

A plurality of individual LED components 21, 22, 23 are incorporatedinto a single device 19 as shown. Each individual LED component of thedevice 19 is structured and operates in the same way as that of FIG. 1.This embodiment may be utilized where a plurality of LEDs are necessaryto provide a desired output from a device and rather than utilizingthree single LEDs fitted individually. The surface mountable nature ofthe device 19 may provide advantages in placement of all LED componentson a suitable substrate and driving mechanism such as a printed circuitboard (PCB) while still co-joined.

Naturally, it will be further appreciated that the number of individualLEDs within the device 19 as shown in FIGS. 4A and 4B can be 2, 3 or anyother number such as shown by way of example in FIG. 1 of Hong Kongpatent application No. 03104219.4 filed Jun. 12, 2003 for A SURFACEMOUNTABLE LIGHT EMITTING DEVICE AND METHOD OF MANUFACTURE which has beenincorporated by reference.

A yet further advantage of the embodiment as shown in FIGS. 4A and 4B isthat different colored LEDs can be provided. For example, during themanufacturing process, a different chip may be fitted to each individualLED component 21, 22 and 23. This may allow, for example, a red, blueand green color arrangement through the use of a different color chip ineach of the individual LED components 21, 22 and 23 so as to provide afull video color spectrum, or the like. It will be appreciated that avariety of other color schemes are possible.

Referring next to FIG. 5A, shown is a schematic of a driving circuit ofan optical device 6 according to an embodiment of the present invention.

Shown is a DC/DC step-up converter 25 having a coil 26, a resistor 27, atransistor 28, a diode 29, a capacitor 30 and an LED 6.

The step up converter (from 1V to 4V) uses +1 VDC to +3 VDC to drive thedevice 6 up to 70 Ma. The coil 26 and the transistor 28 are used as aswitching regulator and the resistor 27 is used as a current control.The diode 29 provides a rectifier and the capacitor provides a ripplefilter. In this case, a single 1.5V battery is utilized to drive the LED6.

Referring next to FIG. 5B, shown is a schematic of a driving circuit ofan optical device 6 according to an alternative embodiment of thepresent invention.

Shown is a light emitting device 1, an LED 6, a photo sensor 31, aresistor 32, a transistor 33 and a resistor 34.

The light emitting device 1 (in this case a surface mountable diodepackage) circuit comprises a photo sensor 31 in die form and the LED 6to which a resistor 32, a transistor 33 and a resistor 34 are connected.This allows for the LED 6 to activate based upon varying light levelsdetected by the photo sensor.

Referring next to FIG. 5C, shown is a schematic of a driving circuit ofan optical device according to an alternative embodiment of the presentinvention utilizing an integrated circuit.

Shown is a light emitting device 1, an LED 6 and an integrated circuitcontrol die 35. The integrated circuit control die 35 is operablybetween the LED 6 and a power source, such as a 1.5 V DC power source,to control operation of the LED 6. The integrated circuit control die 35provides control, for example, for the LED 6 to flash or blink in apattern.

Referring next to FIG. 6, shown is a side cross-sectional view of alight bulb, integrating the device of FIG. 1 into a light bulb housing,according to an embodiment of the present invention.

Shown is a light bulb 36 having the light emitting device 1 of FIG. 1with the lens 10, the printed circuit board (PCB) 37, an E10 lamp base38, the wires 39 to the lamp base 38 and anode 40, an epoxy seal 41 anda glass encasement 42.

The light emitting device 1, which acts as the optical filament of thelight bulb 36, is operably connected to the printed circuit board 37secured at the top of the lamp base 38. Two wires 39 are operablyconnected each to the lamp base 38 and anode 40 to provide power to thelight emitting device 1. The lens 10 has the inverted cone feature shownin, for example, FIG. 1 and is located inside the glass encasement 42 ofthe light bulb 36. The epoxy seal 41 is between the glass encasement 42and the lamp base 38. The light emitting device 1 may also be used inapplications such as exterior and interior automotive lights, whereincircuitry is provided in the printed circuit board to draw off a 2 ampcurrent to drive the flasher circuit of the automobile (or whateveramount of current happens to required for the particular application).

Referring next to FIG. 7, shown is a partial side cross-sectional viewof an optical device according to an alternative embodiment of thepresent invention.

Shown is a light emitting device 1 having a main body portion 2. Themain body portion 2 is a variant of that of FIG. 1 in that itincorporates three semiconductor chips 6, 45, 47 in one surfacemountable light emitting device 1. The light emitting device 1 has alead frame 3, a further contact 4, electrical connections 7, 46, 48, andportions of a compound 8, 9. A lower transfer section 11 of the lens 10of FIG. 1 is also partially shown.

The lead frame 3 is located in the main body portion 2. A plurality(three in this case) of semiconductor chips 6, 45, 47 are mounted on thelead frame 3. A further contact 4 to provide the path for currentthrough the chip 6 is also provided and attached to the semiconductorchips 6, 45, 47 by electrical connections 7, 46, 48 such as a leadbonding wires. Preferably the semiconductor chips are single bond padLEDs, but may also be a double bond pad LEDs (which would require anadditional cathode bonding wire). The lens 10 (partially shown) isprovided over an upper surface to encapsulate the chips 6, 45, 47 andprovide preferred optical characteristics.

As also shown in this particular embodiment, the lens 5 and the mainbody portion 2 may be provided in a single molding step as integrallymolded portions from the same material using a transparent opticalmolded compound. Alternatively, the lens 10 may attach to the device 1using optical grade glue. To act as a lens, the material should besubstantially transparent although not necessarily completelytransparent as there may be some desire to adapt the opticalcharacteristics of the output of the semiconductor chip with the lens.The semiconductor chips 6, 45, 47 are recessed into recesses within thelead frame such that sides of the recesses act as reflectors.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

All references cited herein are herein incorporated by reference.

1. A high power surface mountable light emitting device comprising: a light emitting semiconductor chip; a thermally and electrically conductive lead frame connected to said chip and exposed over a substantial portion of the underside of the device; a lead wire from said chip to a contact exposed at least partially on a side of said device; and a lens over said chip wherein the lens comprises: a lower transfer section; and an upper ejector section situated upon the lower transfer section, said lower transfer section operable for placement upon the light emitting semiconductor chip and operable to transfer the radiant emission to said upper ejector section, said upper ejector section shaped such that the emission is redistributed externally into a substantial solid angle.
 2. The device of claim 1 wherein the lower transfer section is a solid of revolution having a profile in the shape of an ellipse with a long axis parallel to an axis of revolution of the solid and displaced laterally therefrom so as to place the focus of said elliptical profile on the opposite side of said axis.
 3. The device of claim 2 wherein the upper ejector section is a cylinder of the same diameter as a top diameter of the transfer section, said cylinder having a conical depression on its top surface.
 4. The device of claim 2 wherein said lateral displacement substantially equals the radius of said ellipsoid at its focus.
 5. The device of claim 1 wherein said upper ejector section is a conicoid.
 6. The device of claim 1 wherein said upper ejector section is a everted sphere.
 7. The device of claim 1 wherein said upper ejector section is an indented section of a sphere.
 8. The device of claim 4 wherein said upper ejector section is a cylinder.
 9. The device of claim 1 wherein the upper ejector section has a diffusive surface.
 10. The device of claim 1 wherein the lens has a surface with graded sub-wavelength roughness for reflective scattering of said emitted light out of said device.
 11. The device of claim 1 wherein the lens is made of transparent material for distributing the radiant emission of a light emitter, comprising an expander section for receiving said radiant emission and narrowing its angular range to that of light guiding via total internal reflection, and a cylindrical ejector section for receiving said angularly narrowed radiation and ejecting it by means of graded sub-wavelength roughness on its surface.
 12. The device of claim 1 wherein said lens comprises substantially encasement of an upper side of said chip in a transparent compound.
 13. The device of claim 12 wherein said transparent compound forms both a lens and a portion of the body about the lead frame.
 14. The device of claim 13 wherein said transparent compound is keyed into the metallic lead frame and said contact to reduce separation.
 15. The device of claim 1 wherein said device includes a reflector cup about said chip to reflect light from the sides of the chip generally into a direction extending from the upper surface of said chip.
 16. The device of claim 15 wherein said reflector cup is metallic.
 17. The device of claim 15 wherein said reflector cup comprises a core material with a highly reflective metallic coating.
 18. The device of claim 17 wherein said reflective coating comprises chromium or silver plating.
 19. The device of claim 18 wherein said lead frame comprises a substantially copper core with at least one other metal plating on an underside thereof.
 20. The device of claim 19 wherein said metal plating on said underside of said lead frame comprises a plating of solder of palladium.
 21. The device of claim 15 wherein said transparent compound comprises an epoxy resin.
 22. The device of claim 1 wherein said lead wire to said contact is a gold wire. 